Fuser control system

ABSTRACT

A control system in which a first heat source and a second heat source, cooperating with one another, are regulated for maintaining a moving article at a preselected temperature. The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

United States Paten [191 Tsilibes et al.

[ FUSER CONTROL SYSTEM I [75] Inventors: George N. Tsilibes; Frank V.

'DAmico; John G. Hamm, 111, all of Rochester, NY.

[73] Assignee: Xerox Corporation, Stamford,

I Conn.

[22] Filed: Feb. 20, 1973 [2 1] Appl. No.: 334,167

[52] US. Cl 219/216, 219/388, 219/497, 432/227 [51] Int. Cl G03g 15/20 [58] Field of Search 219/216, 388, 497; 250/317-319; 432/227; 118/637 [5 6] References Cited UNITED STATES PATENTS 3,532,855 10/1970 VanCleave ..2l9/216 3,558,853 1/1971 Schluntz 219/216 Primary Examiner-C. L. Albritton Att0rneyJames J. Ralabate et a1.

57 ABSTRACT A control system in which a first heat source and a second heat source, cooperating with one another, are regulated for maintaining a moving article at a preselected temperature.

The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

12 Claims, 7 Drawing Figures PATENTEU BEC25 I973 SHEET 1 OF 5 PATENTED HEB 2 5 I975 SHEET 2 BF 5 PATENTED DEC 2 5 I878 saw u' 0F 5 FIG. 5

FIG. '7

1 FUSER CONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to an electrostatographic printing machine, and more particularly concerns a control system utilized to regulate a fusing apparatus incorporated therein for affixing permanently a powder pattern, in image configuration, to a sheet of support material.

An electrostatographic process involves the formation and utilization of electrostatic latent charge patterns for the purpose of recording and reproducing the patterns in a viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography is a class of electrostatography which employs a photosensitive medium to form, with the aid of radiation, the electrostatic latent charge pattern. Electrography is that class of electrostatography which utilizes an insulating medium to form, without the aid of radiation, the electrostatic latent charge pattern. Fusing, which is the act of affixing permanently a powder image to a sheet of support material, is employed in all of the aforementioned classes of electrostatography. In the illustrated embodiment hereinafter discussed, an electrophotographic process is described.

In electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document to be reproduced. This records an electrostatic latent image on the photoconductive surface. A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the electrostatic latent image. Toner particles are attracted electrostatically to the latent image forming a toner powder image thereon. The toner powder image is, then, transferred to the sheet of support material. Generally, the toner particles include fusible resins. When such toner particles are transferred to the support material the powder image can be permanently affixed thereto by heating. The process of applying heat to the toner powder image partially dissolves the toner particles causing them to fuse into the sheet of support material.

Multi-color electrophotographic printing repeats the foregoing processes of charging, exposing, developing and transferring a plurality of cycles. However, each development cycle deposits differently colored toner particles on the support material, in superimposed registration with the previously deposited layer of toner particles. Hence, the support material will have transferred thereto a multi-layered toner powder image. The multi-layered toner powder image is heated to coalesce and become transparent, i.e., each toner layer modulates the light rays passing therethrough, to form a copy having a single composite color. In this manner, the modulated light rays transmitted through the toner powder images are reflected from the support material back through the toner powder image to the eye of the observer.

In heating the toner powder image, it is preferable to raise the temperature of the support material so that it is substantially the same as the fusing temperature of the toner particles. Thus, the support material functions as a heat source rather than a heat sink during the fusing operation. Hence, a suitable fusing apparatus may include a pair of heat sources, one to heat the support material and the other to heat the toner powder image. Various types of fusing systems have been developed which permit heating of the support material as well as applying radiant heat to the toner particles disposed thereon. For example, a suitable fusing apparatus for use in a multi-color electrophotographic printing machine is described in copending application Ser. No. 300,531, filed in I972. The foregoing fusing apparatus utilizes a radiant energy source and a heated transport for the support material. This type of fusing apparatus will be described hereinafter in conjunction with the control system of the present invention. In operation, the radiant energy source and transport heat source operate in conjunction with one another to coalesce and affix the multi-layered toner powder image to the support material. The foregoing is achieved without charring or igniting the support material. Hence, the radiant energy source and the transport heat source must be independently and cooperatively controlled so as to insure that the temperature of the support material with the multi-layered toner powder image deposited thereon is appropriately regulated.

Accordingly, it is a primary object of the present invention to improve the control system regulating a fusing apparatus arranged to affix permanently single or multi-layered toner powder images onto a sheet of support material.

SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided a control system for regulating a first heat source and a second heat source arranged to cooperate with one another for maintaining a moving article at a preselected temperature.

This is accomplished, in the present instance, by a control system having first control means and second control means. Pursuant to the present invention, the first control means is responsive to the temperature detected in the region of the moving article for regulating the heat energy being generated by the first heat source. In this way, the first heat source is controlled to maintain the moving article at substantially about the preselected temperature. Moreover, the first control means is arranged to compensate automatically for fluctuations in the external voltage being furnished thereto. Further, in accordance with the present invention, the second control means is operatively associated with the first control means and responsive to the temperature detected in the region of the second heat source for regulating the heat energy being generated by the second heat source. Thus, the second heat source cooperates with the first heat source to maintain the moving article at substantially about the preselected temperature. Furthermore, the second control means is also arranged to compensate automatically for fluctuations in the external voltage being furnished thereto.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view of an electrophotographic printing machine having the present invention therein;

FIG. 2 is a perspective view of the fusing apparatus utilized in the FIG. 1 printing machine;

FIG. 3 is a sectional elevational view of the FIG. 2 fusing apparatus;

FIG. 4 is a schematic diagram of the electrical circuitry employed to regulate the radiant heater depicted in the FIG. 2 fusing apparatus;

FIG. 5 is a schematic diagram of the electrical circuitry employed to control the auxiliary heater depictec in the FIG. 3 fusing apparatus;

FIG. 6 is an electrical circuit diagram depicting the electrical components incorporated in the FIG. 4 and FIG. 5 schematic diagrams; and

FIG. 7 is a schematic diagram of an integrated circuit used in the FIG. 6 circuit diagram.

While the present invention will be described in connection with a preferred embodiment, it will be understood that is is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings wherein like reference numerals have been used throughout to designate like elements, FIG. 1 schematically illustrates a multi-color electrophotographic printing machine in which the present invention may be incorporated. The electrophotographic printing machine depicted in FIG. 1 shows the various components utilized therein for producing multi-color copies from a colored original. Although the control system of the present invention is particularly well adapted for use in conjunction with the fusing apparatus depicted in the FIG. 1 electrophotographic printing machine, it should become evident from the following description that it is equally well suited for use in a wide variety of electrostatographic printing machines and is not necessarily limited in its application to the particular embodiment shown herein.

The printing machine depicted in FIG. 1 employs a photoconductive member having a drum 10 mounted rotatably within the machine frame (not shown). Photoconductive surface 12 is mounted on the exterior circumferential surface of drum 10. One type of suitable photoconductive material is disclosed in U. S. Pat. No. 3,655,377 issued to Sechak in l972. Drum 10 rotates in the direction indicated by arrow 14 to move photoconductive surface 12 sequentially through a series of processing stations. A common drive motor (not shown) rotates drum 10 at a predetermined speed relative to the other machine operating mechanisms. The various machine operations are coordinated with one another to produce the proper sequence of events at the appropriate processing stations. The sequencing of events within the electrophotographic printing machine is controlled by the machine logic circuitry. The logic timing may be keyed from individual logic elements, or, in lieu thereof, keyed to the angular rotation of drum 10. In the event that the timing of the various sequencing events is keyed to the angular rotation of drum l0,'a timing disc (not shown), may be mounted at one end of drum 10. Preferably, the timing disc includes a plurality of angularly spaced slits in the periphery thereof. A light source is adapted to transmit light rays through the foregoing slits and energize a photosensor. Therefore, as the various slits permit the passage of light therethrough, the photosensor is actuated 4 thereby energizing the appropriate logic circuitry for initiating various processes at the respective machine stations.

Initially, drum 10 moves photoconductive surface 12 through charging station A. Charging station A has positioned thereat a corona generating device indicated generally at 16. Corona generating device 16 extends in a generally transverse direction across photoconductive surface 12. In this manner, corona generating device 16 is readily able to charge photoconductive surface 12 to a relatively high substantially uniform potential. Preferably, corona generating device 16 is of a type described in US. Pat. No. 2,778,946 issued to Mayo in 1957.

Thereafter, drum 10 is rotated to exposure station B where charged photoconductive surface 12 is exposed to a color filtered light image of the original document. Exposure station B includes thereat a moving lens system, generally designated by the reference numeral 18, and a color filter mechanism, shown generally at 20. A suitable moving lens system is disclosed in U. S. Pat. No. 3,062,108 issued to Mayo in 1962, and a suitable color filter mechanism is described in copending application Ser. No. 830,282, filed in 1969. As shown in FIG. 1, an original document 22, such as a sheet of paper, book, or the like is placed face down upon transparent viewing platen 24. Lamp assembly 26, lens system 18, and filter mechanism 20 are moved .in a timed relation with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. Thus, a flowing light image of original document 22 is projected onto photoconductive surface 12 to record an electrostatic latent image thereof thereon. Filter mechanism 20 is adapted to interpose selected color filters into the optical light path. The appropriate color filter operates on the light rays passing through lens 18 to produce an electrostatic latent image on photoconductive surface 12 which corresponds to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image.

Subsequent to exposure, drum 10 rotates the single color electrostatic latent image recorded on photoconductive surface 12 to development station C. Development station C includes thereat three individual developer units, generally indicated by the reference numerals 28, 30 and 32, respectively. A suitable development station employing a plurality of developer units is disclosed in copending application Ser. No. 255,259, filed in 1972. Preferably, the developer units are all of a type referred to generally as magnetic brush developer units. A typical magnetic brush developer unit utilizes a magnetizable developer mix having carrier granules and toner particles. The developer mix is continually brought through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 12 is developed by bringing the brush of developer mix into contact therewith. Each of the respective developer units contain discretely colored toner particles corresponding to the complement of the spectral region of the wavelength of light transmitted through filter 20, e.g., a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles thereon, blue and red latent images are developed with yellow and cyan toner particles, respectively.

Drum 10, with the toner powder image adhering electrostatically to photoconductive surface 12, is next rotated to transfer station D. At transfer station D, the toner powder image is transferred to a sheet of final support material 34. Final support material 34 may be, amongst others, plain paper or a thermoplastic sheet. A transfer roll, shown generally at 36, is located at transfer station D. Transfer roll 36 recirculates support material 34 and is biased electrically to a potential of sufficient magnitude and polarity to attract toner particles from the latent image recorded on photoconductive surface 12 thereto. A suitably electrically biased transferroll is described in U. S. Pat. No. 3,612,677 issued to Langdon et al. in 1971. Transfer roll 36 rotates, in the: direction of arrow 38, in synchronism with drum I0 in this case at the same angular velocity therewith). Inasmuch as support material 34 is secured releasably onth'e exterior circumferential surface of transfer roll 36' for movement in a recirculating path therewith, successive superimposed toner powder images may be transferred thereto. Support material 34 is advanced from a stack 40 thereof disposed on tray 42. Feed roll 44, in operative communication with retard roll 46, advances and separates the uppermost sheet from stack 40. The advancing sheet moves into chute 48 which directs it into the nip between register rolls 50. Thereafter, gripper fingers 52 mounted on transfer roll 36 secure releasably thereon support material 34 for movement in 'a recirculating path therewith. After a plurality of toner powder images have been deposited on support material 34, support material 34 is separated from transfer roll 36. Gripper fingers 52 release support material 34 permitting stripper bar 54 to separate it from transfer roll 36. After support material 34 is stripped from transfer roll 36, an endless belt conveyor 56 advances support material 34 to fixing station E. At fixing station E, a fusing apparatus, indicated generally at 58, coalesces the transferred powder image to support material 34. Fusing apparatus 58 is of the type hereinbefore mentioned as being described in copending application Ser. No. 300,531 filed in 1972, and will be discussed hereinafter in greater detail with reference to FIGS. 2 and 3. Turning once again to FIG. 1, after the plurality of toner powder images have been affixed permanently to support material 34, support material 34 is advanced by endless belt conveyors 60 and 62 to catch tray 64 for subsequent removal therefrom by a machine operator.

Although a preponderance of toner particles are transferred to support material 34, invariably some residual toner particles remain on photoconductive surface .12. These residual toner particles are removed from photoconductive surface 12 at cleaning station F. As drum moves through cleaning station F, the residual toner particles are first brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on the toner particles. The neutralized toner particles are then mechanically cleaned from photoconductive surface 12 by a rotatably mounted fibrous brush 66. A suitable brush cleaning device is described in U. S. Pat. No. 3,590,412 issued to Gerbasi in 1971. Rotatably mounted brush 66 is positioned at cleaning station F and maintained in contact with photoconductive surface 12. In this manner, residual toner particles remaining on photoconductive surface 12 after each transfer operation are removed therefrom.

It is believed that the foregoing description is sufficient for purposes of the present application to show the general operation of an electrophotographic printing machine embodying the teachings of the present invention.

Referring now to the specific subject matter of the present invention, FIG. 2 depicts a perspective view of fusing apparatus 58 which is used in the electrophotographic printing machine of FIG. 1. Fusing apparatus 58 is depicted in FIG. 2 as having the cover member 72 pivoted to an open position. Conveyor 56 is associated with fusing apparaus 58 to transport support material 34 from transfer roll 36 thereto. Conveyor 56 comprises a plurality of endless belts 68 entrained about a pair of opposed spaced rollers 70. A vacuum system maintains a low pressure by drawing air through apertures 73 of belt 68 to tack support material 34 thereto. Cover member 72 includes radiant energy source 76. Lower housing member 74 defines an open ended chamber having a pair of spaced rollers 78 and 80 mounted rotatably on a transport frame disposed therein. An endless belt 82 is entrained about rollers 78 and 80. Endless belt 82 includes a plurality of apertures 84 therein which are arranged to draw air therethrough such that support material 34 is tacked thereto as it ,passes through fuser 58. Preferably, cover member 72 includes a sheet metal shell having secured to the interior surface thereof suitable insulation. A nylon fiber coating is sprayed on the exterior surface of cover member 72 to protect the operator. An outer reflector is suitably attached to the insulation secured to the interior surface of the cover metal shell. An inner reflector is mounted on the outer reflector. As mounted, the inner and outer reflectors are spaced from one another permitting air to circulate therebetween. Sensing means or thermistor 1100 (FIG. 4) is positioned in the air space between the inner and outer reflectors to .measure the temperature thereat. One type of suitable thermistor for detecting the temperature in the air space between the inner and outer reflectors is a glass bead thermistor.

The radiant energy source or radiant heat strip 76 is, preferably, a nickel chromium alloy ribbon entrained helically about a pair of opposed spaced support members such as ceramic spools. Heat strip 76 is arranged so as to provide substantially uniform radiation. A suitable guide, preferably quartz woven string, is wound over heat strips 76 and adapted to prevent support material 34 from contacting it.

Turning now to FIG. 3, there is shown a sectional view of lower housing assembly 74. Lower housing 74 includes a sheet metal shell having insulation secured to the interior surface thereof. The transport frame is mounted removably in the shell. Rollers 78 and 80 are mounted rotatably on the frame and have entrained thereabout endless belt 82. Interior surface 82b of endless belt 82 is adapted to be closely adjacent to plate member 36. Plate member 86 is adapted to be heated by air moving in the direction of arrows 90. Blower member 92 has a vane member 94 mounted thereon. Vane member 94 is adapted to produce an air flow in the direction of arrows 90. The air flow passes over heating means or auxiliary heater 96 onto plate member 86 raising the temperature thereof. Plate member 86 is closely adjacent to the under surface 82b of endless belt 82 and transmits heat thereto. This, in turn, raises the temperature of support member 34 minimizing any heat lost therefrom. In this manner, radiant energy from heat strip 76, in conjunction with auxiliary heater 96 coalesces the multi-layered toner powder image formed on support material 34.-

Auxiliary heater 96 is, preferably, an 800 watt tubular high mass heater. Sensing means or a can enclosed bead thermistor 118 (FIG. is arranged to detect the temperature of endless belt 82. Bead thermistor 118 is mounted on a thermally conductive shoe which, in turn, is adapted to contact the lower surface 82b of belt 82. Preferably, blower motor 92 is a two-pole split capacitor motor and is adapted to maintain a pressure differential of suitable magnitude to tack support material 34 to the exterior surface 82a of endless belt 82. As hereinbefore mentioned, a suitable timing disc, mounted on drum l0 and adapted to rotate therewith, cooperates with the machine logic to actuate fusing apparatus 58 when sheet 34 passes therein. The operating modes of fusing apparatus 58 are summarized in Table l.

TABLE 1 Machine Operating Auxiliary Radiant Mode Heater Heater Warm up On On Standby On Off No support material On Off print mode in fuser Support material Off On at print mode in fuser, 1750 Watts air temperature less than 440F Support material On On at print mode in fuser. i250 Watts air temperature greater than 440! In operation, the electrophotographic printing machine is energized and heated from a cold condition to a standby condition. During the warm-up phase, both auxiliary heater 96 and radiant energy source 76 are activated. Initially, radiant energy source 76 operates at full power of about 1,750 watts.'When endless belt 82 is raised to a preselected standby condition which may range from about 390F to about 420F, depending upon humidity conditions, radiant energy source 76 is de-energized. Fuser 58 is maintained at the standby temperature by auxiliary heater 96.,When a sheet of support material 34 enters fuser 58, the machine logic energizes radiant energy source 76 at the upper power level (in this case 1,750 watts) and de-energizes auxiliary heater 96. As the sheet of support material 34 exits fusing apparatus 58, the machine control logic energizes auxiliary heater 96 and de-energizes radiant energy source 76. The preceding control cycle continues as long as thermistor 100, positioned in the air space between the outer and inner reflectors, indicates a temperature below about 440F. If, however, the temperature exceeds about 440F, radiant energy source 76 is energized at a lower power level (in this case 1,250 watts) when a sheet of support material 34 enters fusing apparatus 58. In addition thereto, auxiliary heater 96 remains energized. As the sheet of support material 34 exits fusing apparatus 58, the machine control logic de-energizes radiant energy source 76, while auxiliary heater 96 remains energized. Furthermore, in the event that a transparency material such as a coated thermoplastic material rather than a sheet of plain paper is passing through fusing apparatus 58, energy source 76 v is inactivated and auxiliary heater 96 is activated.

Referring now to FIGS. 4 and 5, the functional block diagrams for the control system regulating radiant energy source 76 and auxiliary heating means 96, respectively, are described therein. FIG. 4 depicts schematically the second control means, indicated generally at 98, employed to regulate radiant energy source 76. Second control means 98 is adapted to regulate the energy output from radiant energy source 76. Preferably, second control means 98 includes sensing means or thermistor 100 adapted to detect the temperature in the air space between the outer and inner reflectors in the region of radiant energy source 76. Voltage source 102 is arranged to develop a reference voltage corresponding to a predetermined temperature (in this case 440F). Comparator circuit 104 compares the reference voltage generated by voltage source 102 with the electrical signal developed from thermistor 100. The output from comparator 104 is an electrical error signal indicative of the difference between the reference temperature and the measured temperature in the air space between the inner and outer reflectors. Voltage source 106 develops a reference voltage which is compared with the sum of the error signal from comparator 104 and a voltage proportional to the voltage exciting the radiant energy source 76 in comparator 108. Photomod 110 which comprises an incandescent light source and a cadmium sulphide photocell, is adapted to detect the voltage being developed by the radiant energy source and develops an electrical voltage proportional thereto. The resistance change in the photocell is proportional to the voltage applied to the photocell. In this way, the voltage proportional to that of radiant energy source 76 is summed with the error signal from comparator 104 and compared with the reference voltage developed by voltage source 106 in comparator 108 to develop an electrical error voltage therefrom. The error voltage from comparator 108 is utilized to excite radiant energy source controller 112. Radiant energy source controller 112 is adapted to produce volts RMS when thermistor 100 indicates that the temperature is less than 440F, and 152 volts RMS when thermistor 100 indicates that the temperature is greater than 440F. Radiant energy source controller 112 is preferably, a zero crossover voltage controller and is adapted to generate a voltage of 180 volts RMS when the error signal input thereto from comparator 104 indicates that the temprature in the air space between the outer and inner reflectors is less than 440F. Similarly, radiant energy source 76 is adapted to generate a voltage of 152 volts RMS when the error signal from comparator 104 indicates that the air temperature between the inner and outer reflectors is greater than 440F. Hence, the output from radiant energy source controller 112 is adapted to excite radiant energy source 76 at either a voltage of 180 volts RMS or at a voltage of 152 volts RMS. Radiant energy source 76 is adapted to develop l,750 watts at an excitation of 180 volts RMS,

' and 1250 watts at an excitation of 15 2 volts RMS.

- to 420F. Thermistor 118 disposed against undersurface 82b of endless belt 82 develops an electrical output which is a function of the temperature thereof. The electrical output from thermistor 118 is compared with the voltage from voltage reference 116 in voltage comparator 120. The difference therebetween is an error voltage. Photomod 124 develops a voltage output which is a function of the voltage of auxiliary heater 96. Voltage reference 122 corresponds to a voltage output of about 180 volts RMS. The output from photomod 124 and the error signal from comparator 120 are summed with the voltage output from voltage reference 122 in comparator 126 and the difference therebetween is the error signal utilized to excite auxiliary heater controller 128. Auxiliary heater controller 128 isa proportional zero crossover voltage controller. The controller voltage output remains substantially constant at 180 volts RMS until the error signal from comparator 126 indicates that the temperature of belt 82 is within F of the temperature set point. At this point, the RMS voltage to the auxiliary heater proportionally decreases such that at a set point of 390F, the voltage is 151 volts RMS, while at a set point of 420F, the voltage is 163 volts RMS. If the temperature of belt 82 increases to about 10F above the set point, the voltage input to auxiliary heater 96 is zero. The foregoing is maintained for external line voltage fluctuations ranging from about 254 volts RMS to about 180 volts RMS. The detailed circuitry employed in second control means 98 adapted to regulate radiant energy source 76 and first control means 114 adapted to regulate auxiliary heater 96 is depicted in the circuit diagram of FIG. 6.

Referring now to FIG. 6, the electrical circuitry utilized to implement the functional block diagrams of FIGS. 4 and 5 is depicted therein. A step down transformer (not shown) develops a 24 volt AC output which is applied to pins 132 and 134. I-Ialf wave rectifier 136 and filter capacitor 138 provide a DC level at pin 135 of voltage regulator 140. Voltage regulator 140 maintains a 5 volt output across resistor 142. Thus, volts are maintained across resistors 142 and 144 of the voltage divider network. Capacitor 146 is the filter for the internal power supply of integrated circuit 148-. Resistor 1'50 limits current into pin 135 of integrated circuit 148 and capacitor 152 compensates for the phase shift of the step down transformer. With pins 154, 156 and 158 connected to one another, pin 158 is biased to a voltage level of about 2.9 volts. Pin 160 is connected to voltage divider network resistors 162 and 169 and to photocell 166. The resistance of photocell 166 varies as a function of the light intensity emitted from light source 168. Photocell 166 and light source 168 form photomod 124 (FIG. 5) which is connected across auxiliary heater 96 (FIG. 5) through resistor 170. In this manner, the resistance of photocell 166 is a function of the voltage applied to auxiliary heater 96. Triac gating pulses are developed at pin 139 of integrated circuit 148 when the voltage across auxiliary heater 96 is low and the resistance of photocell 166 is high. No pulses are present when the voltage across auxiliary heater 96 is high and the resistance of photocell 166 is low. The level at which pulses are inhibited is controlled by resistor 169, which is set for a 180 volt RMS input to auxiliary heater 96. Pulse transformer 172 inverts the gate pulse and isolates the controller from the auxiliary heater triac (not shown). The gating circuitry is inhib- 10 ited by applying a positive voltage to pin 141 of integrated circuit 148 through resistors 176 and diode 178. The positive voltage is removed therefrom when the contacts of relay 180 are closed. Resistors 182 and 184 and capacitor 186 in association with unijunction transistor 188 form a relaxation oscillator with a ramp output at test point 190. Resistor 182 is selected so that the period of oscillation is adjusted to about 1 second. Resistor 192 is selected to adjust the amplitude of oscillation to 0.5 volts. Emitter followers 194 and 174 are used for isolation. Resistor 198 limits the current to diode 200 which biases the ramp at about 7.5 volts above reference. Resistor 308 compensates for temperature changes in the output of unijunction transistor 188. The output of the ramp generator is connected to the positive input, i.e. pin 143, of threshold detector 203. Resistors 204, 206 and 208 in association with thermistor 118 (FIG. 5) form a voltage divider network connected to the negative input, i.e., pin 145, of threshold detector 203.v When the voltage at pin 143 of threshold detector 203 is negative with respect to the voltage at pin 145, the output of threshold detector 203 is low. However, when the voltage at pin 143 is positive with respect to the voltage at pin 145, the output of threshold detector 203 is high. The switch point of threshold detector 203 is set with resistor 204. Near the set point, the output of threshold detector 20 is time proportional between the high and low values over a bandwidth of about plus or minus 10F. This bandwidth corresponds to the thermistor temperature variation. Resistors 212 and 214 are used to set a second set point which can be selected by switch 117. The high set point corresponds to 420F and the low set point corresponds to 390F. Resistors 216 and 218 form a voltage divider network which forward biases diode 179 and inhibits integrated circuit 148. Pin 147 of integrated circuit 220 is biased to a positive value by voltage divider network resistors 222 and 224. Resistor 226 provides voltage feedback so as to limit oscillation at the switch point. Capacitor 228 is charged through resistor 230, operatively associated with diode 232, and arranged to discharge through resistor 234, in conjunction with diode 236. When the output of integrated circuit 203 is adjusted such that 0.7 seconds is low and 0.3 seconds is high, capacitor 228 charges to the threshold voltage and the output from integrated circuit 220 is low so as to energize relay 238. Thereupon, a ready signal is sent to the logic of the printing machine. Diode 240 is a steering diode for by-passing the fuser control circuitry. Capacitor 242 is a filter for the internal power supply of integrated circuit 244. Resistor 246 limits the current into pin 149 of integrated circuit 244, and capacitor 248 compensates for the phase shift of the step down transformer. With pins 175, 153 and 155 con nected to one another, pin 155 is biased at about 2.9 volts. Pin 157 of integrated circuit 244 is connected to a voltage divider network formed from resistors 250, 252 and photocell 254. The resistance of photocell 254 varies as a function of the light intensity emitted from light source 256. Light source 256 is connected across radiant energy source 76 (FIG. 4) and together with photocell 254 form photomod (FIG. 4). Light source 256 is connected across radiant energy source 76 through resistor 258. The resistance of photocell 254 varies as a function of the voltage applied to radiant energy source 76. Triac gating pulses are present at pin 159 of integrated circuit 244 when the voltage across radiant energy source 76 is low and the resistance of photocell 254 is high. The level at which pulses are inhibited is controlled by resistor 250 which is set to maintain a voltage of 180 volts RMS across radiant energy source 76. Resistors 260 and 262 raise the bias of pin 151 when the contacts of relay 264 are closed. Resistor 260 is adjusted to furnish a voltage of 152 volts RMS to radiant energy source 76. Resistors 266 and 268 provide positive feedback around the on/off sensing amplifier of integrated circuit 244. Integrated circuit 244 and integrated circuit 148 will be described hereinafter with reference to FIG. 7. The positive feedback around the on/off sensing amplifier of integrated circuit 244 provides hysteresis and reduces half cycling at the set point. Pulse transformer 130 inverts the gate pulses and isolates the controller from the radiant energy triac (not shown). Gating circuitry is inhibited with positive voltage applied topin 161 of integrated circuit 244 through resistor 270. Positive voltage is removed therefrom when the contacts of relay 272 are closed.

Threshold detector 274- has a positive input at pin 163. This input is biased by voltage divider network resistors 276 and 278. Voltage divider network resistors 280, 282 and 284 and thermistor 100 (FIG. 4) are connected to the negative input of threshold detector 274, i.e., pin 165. When the voltage of pin 165 is positive with respect to pin 163, thermistor 100 has a large resistance and the output of threshold detector 274 is low. When pin 165 is negative with respect to pin 163, thermistor 100 has a low resistance and the output of threshold detector 274 is high. Resistor 330 is adapted to provide feedback to reduce oscillation near the set point. When the temperature of thermistor 100 is above 440F. threshold detector 274 has a high output, thereby energizing relay 269. The voltage input to radiant energy-source 76 is then switched to a lower level. Voltage divider network resistors 288 and 290 furnish the appropriate voltage level required to forward bias diode 292 which, in turn, actuates transistor switch 294. Transistor switch 294 is connected in parallel with the contacts of relay 180,-and is arranged to enable integrated. circuit 148 when the signal actuating auxiliary heater 96 is not present. Relays 180, 272 and 238 are arranged toisolate the controller from the machine logic. Capacitors'296 and 298 prevent relay chatter with a 1K hertz input signal. Resistors 300 and 302 limit the current and diodes 304 and 306 function as steering diodes. A suppression network including resistors 310 and 312, operating in conjunction with capacitors 314 and 316, limit the rate of voltage rise across the auxiliary heater and radiant energy triacs so as to prevent the triacs from turning on when voltage transients occur in the external line voltage. Integrated circuits 203, 220 and 274 are operational amplifiers which are utilized in the circuit depicted in FIG. 6 as voltage comparators. Integrated circuits 148 and 244 are illustrated in FIG. 7, and will be described in conjunction therewith.

Turning now to FIG. 7, there is shown therein integrated circuit 148. Integrated circuit 148 is substantially identical to integrated circuit 244. The correspondence between pins is as follows: pin 141 corresponds to pin 16], pin 167 corresponds to pin 169, pin 139 corresponds to pin 159, pin 176 corresponds to pin 173, pin 160 corresponds to pin 157, pin 154 corresponds to pin 175, pin 156 corresponds to pin 153, pin

158 corresponds to pin 155, pin 177 corresponds to pin 179, and pin 135 corresponds to pin 149. Voltage limiter 318 is arranged to clip the voltage applied across pins 135 and 176 to about 1 :8 volts. Power supply 320 consists of a half wave rectifier which develops +6 volts at pin 167 when an external capacitor is connected across pins 167 and 176. Zero crossing detector 322 generates an output pulse when the voltage across pins 135 and 176 is less than 2 volts. On/off sensing amplifier 324 functions as a differential comparator, when pins 158 and 160 have inputs thereto. The output from on/off sensing amplifier 324 is high when pin 160 is more positive than pin 158. Triac gating circuit 326 contains a driver for direct triac triggering. The gating circuit is enabled when all of the inputs thereto are at a high voltage, i.e., the line voltage must be approximately 0 volts, on/off sensing amplifier 324 has a high output, and the output of inverter 328 is high. The output of inverter 328 is high when the voltage at pin 141 is less than 2 volts.

By way of example, Table 2 presents a summary of the preferred values for the resistance and capacitance elements of FIG. 6. The table summarizes the nominal values of the capacitors and resistors utilized therein by reference numeral.

TABLE 2 Nominal Nominal Resistor Value Value No. Ohms Capacitor Microfarads 142 249 146 144 392 152 0.56 1K 186 33 162 3.9K 228 33 169 25K 242 100 170 6K 248 0.56 176 10K 296 0.47 182 50K 298 0.47 184 10K 314 0.1 192 50K 316 0.1 198 390 204 50K 206 15K 208 15K 212 50K 214 12K 216 1.2K 218 6.8K 222 10K 224 10K 226 100K 230 100K 234 470K 246 1K 250 10K 252 5.6K 258 6K 260 1 OK 262 3.3K 266 18K 268 12K 270 [OK 276 22K 278 22K 280 25K 282 10K 284 10K 288 1K 290 2.7K 300 22 302 22 308 390 310 100 312 100 330 820K 332 15K in recapitulation, first control means is responsive to the temperature detected in the region of the moving article for regulating the energy being generated by the first heat source so as to maintain the moving article at .13 substantially about: thepreselected temperature. The rmoving-article, in thefprinting machine, is a sheet of .support material moving along an endless conveyor belt. The first heat source is an auxiliary heater adapted to heat the conveyor belt on which the support material is'moving. Similarly, second control means is responsive=to.the temperature detected in the region of the second heat source, or: inv this case the radiant energy source, for regulating the heat energy being generated therefrom. The radiant energy source cooperates with 1 the auxiliary heater tomaintain the moving article, i.e., i thesheet of support material, at substantially about the predetermined temperature for affixing a multi-layered toner powder image thereto. Hence, the control system -of 'the.present invention isadapted to regulate the fusing apparatus incorporated in an electrophotographic fprin'ting machine to permanently affix single or multilayered toner powder images to a sheet of support matterial.

Thus, it is apparent that there has been provided, in accordance with this invention, a control system that "fully satisfies the objects, aims and advantages set forth :abovefWhile the invention has been disclosed in conjunction with a specific embodiment, it is evident that many alternatives, modifications and variations will be :apparent to those skilled in the art in light of the fore- .going description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

What is claimed is: l. A control system for regulating a first heat source and a second heat source arranged to cooperate with oneanother in order to maintain a moving article at a :preselected temperature, including:

first control means, responsive to the temperature detected in the region of the moving article, for regulating'the heat energy being generated by the first heat source so as to maintain the moving articleat substantially about the preselected temperature, said first control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto; and second control means, operatively associated with said first control means and responsive to the temperature detected in the region of the second heat source, for regulating the heat energy being generated by the second heat source so that the second heat source, cooperating with the first heat source, maintains the moving article at substantially about the preselected temperature, said second control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto. 2. A control system as recited in claim 1, wherein said first control means includes:

means for sensing the temperature in the region of the moving article and generating an electrical output signal corresponding thereto;

means for developing a reference corresponding to the preselected temperature, said developing means being adapted to generate one of a plurality of discrete references, each reference correspond ing to a discrete preselected temperature;

means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween;

means for generating a reference voltage;

'means for measuring the voltage exciting the first heat source and producing a voltage output indicative thereof;

means for'comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of the fluctuations in the external voltage being furnished to said first control means; and

means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the first heat source.

3. A control system as recited in claim 2, wherein said excitation voltage producing means includes a proportional zero crossing voltage controller arranged to generate a substantially constant RMS voltage in response to said sensing means indicating that the temperature of the moving article is beneath a first predetermined temperature and a substantially zero RMS voltage in response to said sensing means indicating that the moving article is at a temperature greater than a second predetermined temperature, said controller being adapted to adjust the RMS voltage in response to said sensing means indicating that the temperature of the moving article is intermediate the first and second predetermined temperatures to regulate the energy output from the first heat source.

4. A control system as recited in claim 3, wherein said controller is adapted to regulate the RMS voltage in response to said sensing means indicating a temperature change in the region of the moving article with the temperature thereof being intermediate the first and second predetermined temperatures, said controller adjusting the'RMS voltage thereof in an inverse relationship with the temperature change indicated by said sensing means.

5. A control system as recited in claim 1, wherein said second control means includes:

means for sensing thetemperature in the region of the second heat source and generating an electrical output signal corresponding thereto;

means for developing a reference corresponding to the preselected temperature;

means for comparing the electrical output signal from said sensing means with the reference from said developing means to. generate an error signal indicative of the difference therebetween;

means for generating a reference voltage;

means for'measuring the voltage exciting the second heat source and producing a voltage output indicative thereof;

means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of fluctuations in external voltage being furnished to said second control means; and

means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the second heat source.

'6. A control system as recited in claim 5, wherein said excitation voltage producing means includes a controller arranged to generate a substantially constant RMS voltage at a first predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heating source is beneath a predetermined temperature and a substantially constant voltage having a second predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heat source is greater than the predetermined temperature, said controller being adapted to generate a voltage of the first predetermined amplitude when sald sensing means indicates that the temperature in the region of the second heat source is increas ing from a temperature less than the predetermined temperature to a temperature greater than the predetermined temperature,

and said controller being adapted to generate a voltage having the second amplitude when said sensing means indicates that the temperature in the region of the second heat source is decreasing from a temperature greater than the predetermined temperature to a temperature less than the predetermined temperature so as to regulate the energy output from the second heat source. v

7. An electrostatographic printing machine of the type having a fusing apparatus wherein a first heat source and a second heat source cooperate with one another to regulate the temperature of a sheet of support material having a toner powder image deposited thereon at a preselected temperature as the support material passes therethrough, including:

first control means responsive to the temperature detected in the region of the support material for regulating the heat energy being generated by the first heat source so as to maintain the support material at substantially about the preselected temperature, said first control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto; and

second control means, operatively associated with said first control means and responsive to the temperature detected in the region of the second heat source for regulating the heat energy being generated by the second heat source so that the second I heat source cooperating with the first heat source maintains the support material at substantially about the preselected temperature, said second control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto.

8. A printing machine as recited in claim 7, wherein said first control means includes:

means for sensing the temperature in the region of the support material and generating an electrical output signal corresponding thereto;

means for developing a reference corresponding to the preselected temperature, said developing means being adapted to generate one of a plurality of discrete references, each reference corresponding to a discrete preselected temperature;

means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween;

means for generating a reference voltage;

means for measuring the voltage exciting the first heat source and producing a voltage output indicative thereof;

means for comparing the sum of the voltage output and error signal with the reference voltage for genof the fluctuations in external voltage being furnished to said first control means; and

means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the first heat source.

9. A printing machine as recited in claim 8, wherein said excitation voltage producing means includes a proportional zero crossing voltage controller arranged to generate a substantially constant RMS voltage in response to said sensing means indicating that the temperature of the support material is beneath a first predetermined temperature and a substantially zero RMS voltage, in response to said sensing means indicating that the support material is at a temperature greater erating an error voltage substantially independent than a second predetermined temperature, said controller being adapted to adjust the RMS voltage in response to said sensing means indicating that the tem perature of the support material is intermediate the first and second predetermined temperatures to regulate the energy output from the first heat source.

10. A printing machine as recited in claim 9, wherein said controller is adapted to regulate the RMS voltage in response to said sensing means indicating a temperature change in the region of the support material with the temperature thereof being intermediate the first and second predetermined temperatures, said controller adjusting the RMS voltage thereof in an inverse relationship with the temperature change indicated by said sensing means.

11. A printing machine as recited in claim 7, wherein said second control means includes:

means for sensing the temperature in the region of the second heat source and generating an electrical output signal corresponding thereto;

means for developing a reference corresponding to a predetermined temperature; I means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween; means for generating a reference voltage; means for measuring the voltage exciting the second heat source and producing a voltage output indica tive thereof;

means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of fluctuations in external voltage being furnished to said second control means; and

means, responsive to the error voltage, for producing an excitation voltage. adapted to regulate the energy output from the second heat source.

12. A printing machine as recited in claim 11, wherein said excitation voltage producing means includes a controller arranged to generate a substantially constant RMS voltage at a first predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heating source is beneath a predetermined temperature and a substantially constant voltage having a second predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heat source is greater than the predetermined temperature, said controller being adapted to generate a voltage of the first predetermined amplitude when said sensing means indicates that the temperature in the region of the second heat source is increasing from a temperature less than the predetermined temperature to a temperature greater than the predetermined temperature, and said controller being adapted to generate a voltage having the second amplitude when said sensing means indicates that the temperature in the region of heat Sourcethe second heat source is decreasing from a tempera- 

1. A control system for regulating a first heat source and a second heat source arranged to cooperate with one another in order to maintain a moving article at a preselected temperature, including: first control means, responsive to the temperature detected in the region of the moving article, for regulating the heat energy being generated by the first heat source so as to maintain the moving article at substantially about the preselected temperature, said first control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto; and second control means, operatively associated with said first control means and responsive to the temperature detected in the region of the second heat source, for regulating the heat energy being generated by the second heat source so that the second heat source, cooperating with the first heat source, maintains the moving article at substantially about the preselected temperature, said second control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto.
 2. A control system as recited in claim 1, wherein said first control means includes: means for sensing the temperature in the region of the moving article and generating an electrical output signal corresponding thereto; means for developing a reference corresponding to the preselected temperature, said developing means being adapted to generate one of a plurality of discrete references, each reference corresponding to a discrete preselected temperature; means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween; means for generating a reference voltage; means for measuring the voltage exciting the first heat source and producing a voltage output indicative thereof; means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of the fluctuations in the external voltage being furnished to said first control means; and means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the first heat source.
 3. A control system as recited in claim 2, wherein said excitation voltage producing means includes a proportional zero crossing voltage controller arranged to generate a substantially constant RMS voltage in response to said sensing means indicating that the temperature of the moving article is beneath a first predetermined temperature and a substantially zero RMS voltage in response to said sensing means indicating that the moving article is at a temperature greater than a second predetermined temperature, said controller being adapted to adjust the RMS voltage in response to said sensing means indicating that the temperature of the moving article is intermediate the first and second predetermined temperatures to regulate the energy output from the first heat source.
 4. A control system as recited in claim 3, wherein said controller is adapted to regulate the RMS voltage in response to said sensing means indicating a temperature change in the region of the moving article with the temperature thereof being intermediate the first and second predetermined temperatures, said controller adjusting the RMS voltage thereof in an inverse relationship with the temperature change indicated by said sensing means.
 5. A control system as recited in claim 1, wherein said second control means includes: means for sensing the temperature in the region of the second heat source and generatIng an electrical output signal corresponding thereto; means for developing a reference corresponding to the preselected temperature; means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween; means for generating a reference voltage; means for measuring the voltage exciting the second heat source and producing a voltage output indicative thereof; means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of fluctuations in external voltage being furnished to said second control means; and means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the second heat source.
 6. A control system as recited in claim 5, wherein said excitation voltage producing means includes a controller arranged to generate a substantially constant RMS voltage at a first predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heating source is beneath a predetermined temperature and a substantially constant voltage having a second predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heat source is greater than the predetermined temperature, said controller being adapted to generate a voltage of the first predetermined amplitude when saId sensing means indicates that the temperature in the region of the second heat source is increasing from a temperature less than the predetermined temperature to a temperature greater than the predetermined temperature, and said controller being adapted to generate a voltage having the second amplitude when said sensing means indicates that the temperature in the region of the second heat source is decreasing from a temperature greater than the predetermined temperature to a temperature less than the predetermined temperature so as to regulate the energy output from the second heat source.
 7. An electrostatographic printing machine of the type having a fusing apparatus wherein a first heat source and a second heat source cooperate with one another to regulate the temperature of a sheet of support material having a toner powder image deposited thereon at a preselected temperature as the support material passes therethrough, including: first control means responsive to the temperature detected in the region of the support material for regulating the heat energy being generated by the first heat source so as to maintain the support material at substantially about the preselected temperature, said first control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto; and second control means, operatively associated with said first control means and responsive to the temperature detected in the region of the second heat source for regulating the heat energy being generated by the second heat source so that the second heat source cooperating with the first heat source maintains the support material at substantially about the preselected temperature, said second control means being arranged to compensate automatically for fluctuations in the external voltage being furnished thereto.
 8. A printing machine as recited in claim 7, wherein said first control means includes: means for sensing the temperature in the region of the support material and generating an electrical output signal corresponding thereto; means for developing a reference corresponding to the preselected temperature, said developing means being adapted to generate one of a plurality of discrete references, each reference corresponding to a discrete preselected temperature; means for comparing the electrical output signal from said sensing means with the reference from said develoPing means to generate an error signal indicative of the difference therebetween; means for generating a reference voltage; means for measuring the voltage exciting the first heat source and producing a voltage output indicative thereof; means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of the fluctuations in external voltage being furnished to said first control means; and means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the first heat source.
 9. A printing machine as recited in claim 8, wherein said excitation voltage producing means includes a proportional zero crossing voltage controller arranged to generate a substantially constant RMS voltage in response to said sensing means indicating that the temperature of the support material is beneath a first predetermined temperature and a substantially zero RMS voltage in response to said sensing means indicating that the support material is at a temperature greater than a second predetermined temperature, said controller being adapted to adjust the RMS voltage in response to said sensing means indicating that the temperature of the support material is intermediate the first and second predetermined temperatures to regulate the energy output from the first heat source.
 10. A printing machine as recited in claim 9, wherein said controller is adapted to regulate the RMS voltage in response to said sensing means indicating a temperature change in the region of the support material with the temperature thereof being intermediate the first and second predetermined temperatures, said controller adjusting the RMS voltage thereof in an inverse relationship with the temperature change indicated by said sensing means.
 11. A printing machine as recited in claim 7, wherein said second control means includes: means for sensing the temperature in the region of the second heat source and generating an electrical output signal corresponding thereto; means for developing a reference corresponding to a predetermined temperature; means for comparing the electrical output signal from said sensing means with the reference from said developing means to generate an error signal indicative of the difference therebetween; means for generating a reference voltage; means for measuring the voltage exciting the second heat source and producing a voltage output indicative thereof; means for comparing the sum of the voltage output and error signal with the reference voltage for generating an error voltage substantially independent of fluctuations in external voltage being furnished to said second control means; and means, responsive to the error voltage, for producing an excitation voltage adapted to regulate the energy output from the second heat source.
 12. A printing machine as recited in claim 11, wherein said excitation voltage producing means includes a controller arranged to generate a substantially constant RMS voltage at a first predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heating source is beneath a predetermined temperature and a substantially constant voltage having a second predetermined amplitude in response to said sensing means indicating that the temperature in the region of the second heat source is greater than the predetermined temperature, said controller being adapted to generate a voltage of the first predetermined amplitude when said sensing means indicates that the temperature in the region of the second heat source is increasing from a temperature less than the predetermined temperature to a temperature greater than the predetermined temperature, and said controller being adapted to generate a voltage having the second amplitude when said sensing means indicates that the temperature in the region of the second heaT source is decreasing from a temperature greater than the predetermined temperature to a temperature less than the predetermined temperature so as to regulate the energy output from the second heat source. 